The present invention relates to high-efficiency, small-scale, combined heat and power (CHP), concentrating solar energy systems, and to a radio-dial drive, for celestial tracking mechanisms, which may be used with the concentrating solar energy systems.
Solar photovoltaic collectors are usually of the flat plate type, consisting of a large area of stationary photovoltaic cells that receive natural sunlight, but do not follow the apparent motion of the sun. As yet, power generation by these photovoltaic collectors is not economic, when compared with conventional power sources of fossil fuels.
One approach to reducing the cost of power generation by solar photovoltaic collectors is to concentrate the sunlight by optical means, such as lenses and (or) mirrors, thus reducing the actual area of the photovoltaic cells, per kW. Current cells can be activated by solar radiation, which is concentrated by a factor of up to 1,000; therefore the required photovoltaic cell area per kW is 1,000 times less. The resultant power system may be more cost-effective, even when taking into account the more expensive photovoltaic cells for the concentrated radiation and the additional components of the concentrating system, such as, the focusing optics, the mechanical support structure, the tracking mechanism, and the computer control for the tracking.
Large concentrating solar power systems, operating with photovoltaic cells, have been proposed and built. These may include a single, large concentrator, or a cluster of large concentrators. Yet, to be economically viable, a collector area of about 100 to 200 square meters is required, for each concentrator, so as to spread the cost of the additional components of the concentrating system, per kW. These large concentrating solar power systems are suitable for remote areas, which have no access to the grid.
Nonetheless, employing large concentrating solar power systems suffers from a number of drawbacks.    1. Although in general, a larger system tends to be more economical than a smaller system, there are disadvantages to a larger system, as well. Wind resistance is higher, creating high forces on the collector, and these may lead to structural deformation and may interfere with the accuracy of the tracking.
In consequence, the support structure and tracking mechanism must be massive and quite expensive.    2. With an efficiency of power conversion to photovoltaic cells in the range of 10 to about 37 percent, most of the solar energy is discharged as heat. Yet in a centralized, remote area there is little opportunity to utilize that heat, for example, in a Combined Power and Heat (CPH) system, thus the heat is wasted.    3. The large concentrating solar power systems are installed away from the consumer. Therefore additional costs due to power distribution and to transmission losses of about 10-20% of the power transmitted are incurred, raising the cost of the electric power thus produced by factors of between 2 and 3. Yet with large concentrating systems of photovoltaic cells, transmission and distribution costs cannot be avoided since the systems are too large to be installed at the points of consumption.    4. The initial investment for large concentrating solar power systems is very high, making decisions in this regard difficult, bureaucratic, and risky.    5. Furthermore, competitive costs may be realized for large concentrating solar power systems if a significant number of them, equivalent for example, to at least 50 megawatt per year, is manufactured. Yet such a market volume is difficult to guarantee; therefore, the investment and the risk associated with the development of such large systems are very high.    6. Large concentrating solar power systems must be installed by trained personnel with specialized equipment and facilities, requiring special contractors, and special licenses, which increase their costs.    7. Large concentrating solar power systems would generally require environmental studies and permits, so as to further increase their costs.    8. Centralized power plants in general are vulnerable to malfunction and sabotage. A single incident of this nature can disrupt power supply for a very large segment of the population.
Ali, A. M., et al. in “A simplified sun tracker for residential applications,” EDB 86-16 86:126758 8607012784 NDN-168-0431-1885-2, 1986, CONF-860222, Pergamon Press, Elmsford, N.Y., USA, describes a sun tracker of low cost and a simple circuitry, that guarantees easy maintenance and operational procedures, making it suitable for domestic applications. The tracker has a shaft encoder, a 50 W motor developing 20 N-m and is provided with a brake, to produce a torque of up to 500 N-m, when the motor is off, together with an electronic control circuit. The system has been tested in conjunction with a domestic concentrator-type solar water heater, but it is believed that maximum benefit would be realized when used as a hybrid system, incorporating photovoltaic and hot water units.
However, the system of Ali, et al. has digital tracking, with a resolution of about 0.72 degrees, relies on polished aluminum troughs, as linear concentrators, and uses a single-axis tracking mechanism, so that overall, it achieves a concentrating factor of only about 10.
Additionally, the system of Ali, et al. uses both photovoltaic cells and hot water units. In practice, the hot water units are not necessary, since waste heat from the photovoltaic converter can be used for producing hot water. Additionally, Komp, R. J. in “Field experience and performance evaluation of a novel photovoltaic thermal hybrid solar energy collector,” EDB, 86-15, 86:116025, 8606508853, NDN-68-0430-1210-7, 1985, CONF-850604, SESCI, Ottawa, Ontario, Canada, describes a new design of a hybrid solar module, capable of furnishing 150 watts (AM1 peak power) of electrical power and 1600 watts of thermal energy in the form of hot water. The module incorporates photovoltaic cells, encapsulated in silicone mounted on the front surface of extruded aluminum fins. Copper tubes forced into the backs of the fins carry the cooling fluid (usually water) to remove the heat while curved aluminum reflectors concentrate light onto the silicon solar cells. The linear curved concentrators (similar to those developed by Winston) require no tracking or seasonal adjustment at the low concentration ratio of 2.1 to 1. The module is intended for residences or small businesses that do not have ready access to conventional utilities. It is essentially a single-size unit and may be installed in a similar manner as a conventional solar heater, with a portion of the solar cell array dedicated to powering and controlling the circulating pump and, if necessary, a valve for a hot water system. The photovoltaic array is split into two separate sections that can be wired in parallel for 12V or in series for 24V systems.
Yet, the system of Komp relies on linear concentrators, does not use tracking, and achieves a concentration ratio of only about 2.1 to 1.
Furthermore, O'Neill, M. J., et al., in “Fabrication, installation, and two-year evaluation of a 245 square meter linear Fresnel lens photovoltaic and thermal (PVT) concentrator system” Dallas/Ft. Worth (DFW) Airport, Texas, DOE/ET/20626-T1, Final technical report, Phase II and Phase III EDB 85-10 85:066882 8505013921 NDN-168-0406-6186, 1985, summarizes the results of the fabrication, installation, and two-year evaluation of the first linear Fresnel lens photovoltaic and thermal (PVT) concentrator system ever deployed. The system is located on the Central Utility Plant at DFW Airport, Texas. The roof-mounted collector field provides 245 square meters of sun-tracking collector aperture area. The nominal 25 kilowatt peak electrical output of the system is used for plant lighting, while the nominal 120 kilowatt peak thermal output is used to preheat domestic water for the nearby AMFAC hotel. The system has performed efficiently and reliably over the full two-year operational period. Long-term system conversion efficiencies have been 7.7% sunlight-to-electricity, 39.1% sunlight-to-heat, 46.8% sunlight-to-total energy output. Each of these efficiency levels is thought to be the highest ever achieved by a commercial-scale photovoltaic system. System durability has also been excellent, with no detectable degradation in performance over the full operational period. In summary, the successful application experiment has verified the potential of the linear Fresnel lens PVT system to reliably and efficiently deliver electricity and heat in commercial-scale applications.
However, the system of O'Neill, et al. is relatively large, producing 120 kilowatt peak thermal output and using 245 square meters of sun-tracking collector aperture area; therefore, it is not applicable to domestic and other small-scale or rooftop applications.
Moreover, Henry, E. M. et al., in “Mississippi County Community College solar photovoltaic total energy project,” EDB 81-11 81:055203 8103063734 NDN-168-0268-2847-2, 1979, CONF-790541-(Vol. 3), Pergamon Press Inc., Elmsford, N.Y., describes a project, by which Mississippi County Community College at Blytheville, Ark., was to derive its electrical and thermal energy from an actively cooled photovoltaic system, developed under the management of TEAM, Inc. The 320 kW concentrator system (DOE standard conditions) was the world's largest photovoltaic demonstration, as of that date (1979). The single-axis tracking collectors were 7 foot by 20 foot parabolic troughs with a geometric concentration of 42. The solar cells were single crystal silicon, designed to match the physical and spectral parameters of the collector. The power conditioning system was utility interactive to provide not only for backup power, but for a power exchange between the solar energy system and the local utility. Process control included data acquisition on all system components and building demands, as well as required control of all components. Thermal energy from the solar cell coolant was provided to the college for winter heating and year-round domestic hot water. The energy system was expected to be operational in the winter of 1979, with connection to the college facilities in the summer of 1980.
However, again the system of Henry, et al. is relatively large, single-axis tracking system, adapted for producing 320 kW, and achieving a concentration factor of only about 42.
Yet there is still a widely recognized need for, and it would be highly advantageous to have, relatively small cost-effective solar power systems, which have a low production rate threshold for competitive mass production, can be installed close to the consumers of energy, and provide means to use the generated heat as well as the electricity.